The invention relates to a method for reducing the concentration of nitrogen oxides in waste gases released during the production of organic amino compounds, wherein an organic compound is first reacted with NOx and/or nitric acid to form an organic nitro compound with the formation of an NOx-containing waste gas stream and the organic nitro compound is converted to the organic amino compound by means of a hydrogen-containing reaction gas, the reaction of the organic nitro compound with the hydrogen-containing reduction gas taking place wish the formation of a hydrogen-containing waste gas stream.
Amines are typical precursors of isocyanates, which are used in the production of formulations for PUR/PIR foams. They are produced by nitration of hydrocarbons, followed by catalytic reduction with hydrogen. In the production of aromatic amines, the nitration of the aromatics used as starting substance is generally carried out with nitric acid. Nitrogen oxides such as NO, NO2 and also N2O, referred to below for the sake of simplicity as NOx, for which very low emission limits have to be observed, are obtained as a by-product here as a result of oxidation reactions. These waste gases therefore have to be treated before they can be released into the environment.
The thermal reduction of nitrogen oxides with natural gas is known from the fuel staging process and is described in Kolb, T., Jansohn, P., & Leuckel, W. (1988). Reduction of NOx Emissions in Turbulent Combustion by Fuel-Staging/Effects of Mixing and Stoichiometry in the Reduction Zone, Proceedings of the Combustion Institute, 22, p. 1193-1203, and in Greul, U. (1998), Experimentelle Untersuchung feuerungstechnischer NOx-Minderungsmaβnahmen bei der Kohlenstaubverbrennung. Düsseldorf: VDI Verlag GmbH, pages 140-145. In this process, natural gas is generally added to a flue gas that has significant concentrations of NO,. The nitrogen oxides obtained during combustion are converted to molecular nitrogen and intermediate components (HCN and NH3) here by adding a fuel (generally methane, natural gas or coal) under reducing conditions. Burnout then takes place by a further addition of combustion air. Depending on process control, the optimum process conditions, such as the degree of the reducing conditions relative to the required residence time, vary with air-fuel ratios of 0.7 to 0.95 being established in the reduction zone in relation to the main combustion zone (Greul, 1998, see above).
Another method for the treatment of waste gases from nitration is oxidation with air and parallel absorption in water of the NO2 formed to give aqueous nitric acid (Reference Document on Best Available Techniques for the Manufacture of Large Volume Inorganic Chemicals—Ammonia, Acids and Fertilisers, European Commission, August 2007, pages 97-99, 117-120, 135-136). This process is used for the production of nitric acid on an industrial scale, hut is costly and, because of the volatile compounds comprised in the waste gas from nitration, is only suitable when combined with an oxidising combustion. The nitric acid obtained is advantageously recycled into the nitration process.
A less technically complex alternative is absorption in more chemically reactive systems, such as dilute sodium hydroxide solution, but: this has the disadvantage that an additional waste substance is obtained which requires treatment.
Furthermore, the selective non-catalytic reduction (SNCR) of nitrogen oxides with ammonia or urea at temperatures of 800-1100° C. is known (Reference Document on Best Available Techniques for Large Combustion Plants, European Commission, July 2006, pages 106-114, 116). However, this method cannot be used efficiently for the waste gases from nitration in question since, with the high NOx concentrations in the waste gases, the exothermicity of the reduction reaction, e.g. according to the following reaction equation4 NO+4 NH3+O2->4 N2+6 H2O,requires a multi-stage reduction with intermediate cooling. In addition, the desired limit values cannot be reliably met because of the limited efficiency of the SNCR method. As a result of the high exothermicity and the associated temperature increase, undesirable secondary reactions, such as the reaction 4 NH3+5 O2→4 NO+6 H2O, gain significance and mean that this method cannot be applied efficiently. This method can therefore only be used for low NOx concentrations but not for waste gases from nitration.
Finally, it is known that tail gases occurring during nitric acid production, which still comprise traces of NOx, can be reduced catalytically with ammonia. This method is referred to as “Selective Catalytic Reduction” (SCR). In order to obtain adequate catalytic activity, the catalyst has to be operated at an elevated temperature. It should be home in mind that the reduction is strongly exothermic, and so this technique has to be used with the customary adiabatic fixed bed reactors only at NOx concentrations that are significantly lower than those in waste gases from a nitration process. A multi-stage execution with intermediate cooling is costly in terms of apparatus and is therefore uneconomical for the present application.
The object of the present invention consisted in providing a more cost-effective method and apparatus for the at least partial elimination of nitrogen oxides that are formed during the production of nitroaromatics.